The first skull shown (with the green ‘eyeball’ ) is that of a cow not a horse. Different shape and no teeth in top front (compare with the other skulls shown).
Now that you mention it, it does appear to be a cow skull! But even if it’s a bit non-traditional, I think it’s still a pretty good Mari Lwyd, so I’m going to leave it in place for now. But I’ve updated the text to me more skull-inclusive. Thanks for catch!